The invention relates to a semiconductor structure configured as a semiconductor switch that can be used in various forms to switch currents. Bidirectional switches of this kind are used for various circuits (I converters, matrix converters) usually at voltage levels, which are higher than those of gate circuits, of up to several thousand volts reverse voltage or blocking voltage.
The object of the invention is to create a monolithic switch that does not need to be constructed in a hybrid manner using several components, which is particularly intended to reduce the static forward and reverse power losses and to provide facilities for reducing switching losses as compared to known switches. In its most distinct form it permits realization of bidirectional switches, i.e. of switches which, at a corresponding polarity of the voltage applied, are capable turning currents on and off in both directions.
The proposed structure requires no pn junction which absorbs (blocks) the blocking voltage or reverse voltage. It rather distinguishes itself in that zones are created, the potential and thus the charge carrier concentration of which may be adjusted by control surfaces (gates). Symbolically speaking this might be considered as a xe2x80x9cgate-controlled dopingxe2x80x9d, which is adjusted by means of the gate voltage in dependence upon the polarity of the voltage at the terminal electrodes (of the main current path) and the desired state of operation.
In one embodiment, the on-state is achieved and maintained by controlling the concentration of charge carriers in the active region in an increasing sense. The controlled fine structures are influenced by fields of MOS structures in such a manner that the concentration in the active region may be enhanced over a wide area, when a first polarity of the control voltage has been selected for the MOS structure. In another embodiment, the off-state and turn-off operation are provided by wherein the control of the concentration entails the reduction of said charge carrier concentration in the active region. In this case the fine structures are influenced by the same MOS structure in such a manner that, when the polarity of the control voltage is inverted, the charge carriers are removed from the active region. These embodiments may be combined.
In other aspects, field plates in the active region on both sides equalize the field characteristics in the off-state. The two sides need not necessarily be located opposite each other on the one or other side of a crystal, they may also be disposed as xe2x80x9cboth sidesxe2x80x9d on the same side of the lightly doped or non-doped semiconductor crystal. The latter case refers to a so-called a lateral switching element which requires photo lithography on one side only during manufacture.
The fine structures of the invention may be raised above the surface of the semiconductor crystal, they may as well be lowered into the active region of the semiconductor crystal, wherein they may, when lowered, be oriented vertically as well as horizontally. The fine structures remain, regardless of how they are specifically arranged, at the surface or at least close to the surface relative to the semiconductor crystal. They are substantially evenly distributed over this surface and substantially uniformly configured.
The invention may also be employed with a semiconductor device which comprises a pn junction supplying charge carriers, which is however not suitable for absorbing the reverse voltage; in this case the switching blocking operation is possible in one direction only.
The control of the fine structures distributed in the active region over a large area may be effected on one side or both sides by means of the MOS structures, the length of which being a multiple of its width. The narrower the fine structures, the more readily a monolateral MOS structure is capable of influencing the control region in the same manner as a bilateral MOS structure.
Using the invention it is possible to produce bidirectional switches with a highly reduced thickness of the lightly doped zone. In conventional, bidirectionally blocking components, such as thyristors, a first pn junction which blocks a polarity must be supplemented by a second one which is capable of absorbing the reverse voltage of the other polarity. In this manner pnp structures are created, which require at least the double component thickness while having the same blocking capacity. In accordance with the invention the same lightly doped zone is capable of absorbing the blocking voltage as well as the reverse voltage. Due to the low thickness of the main crystalline area lower forward voltages may also be achieved according to the invention.
By controlling the gate voltages of the MOS structures at the xe2x80x9cfine structuresxe2x80x9d opposite the respective adjacent terminal surface, the level of the excess concentration may be adjusted during operation. Thereby minimizing the losses for the respective operation.
The capacities contributing to the control capacities from control surface GA to the opposite terminal electrode KB and on the other side from control surface GB to the opposite terminal electrode KA may be kept relatively small in accordance with the ratio of the structure width to the distance between two fine structures so that the control effort is kept low.
By means of a partial discharge of the component effected prior to turning-off, the turn-off losses may be significantly reduced as compared to those of thyristors capable of being turned off (e.g. a GTO).
The low requirements to the process of manufacture are of particular significance. The component is virtually completely composed of unchanged semiconductor starting material. No pn junctions capable of blocking no doping concentrations to be precisely adjusted and no recombination centers are required. Solely contact areas, from which electrons and holes (as first and complementary second charge carriers) may be injected or drained off, respectively, are provided. These may be highly doped areas, which are short circuited to each other via a terminal surface, which are however areas of short extension transverse to the surface. They may however also be suitable Schottky contacts.
The by far prevailing active region of the semiconductor crystal is covered with an insulating layer and screened by conductive field plates, which equalize the field across the semiconductor crystal in the blocking state. The reduced technological effort of manufacture is of particular advantage especially for semiconductor materials, in which doping is extremely difficult to achieve, such as silicon carbide.